Josh Powell told his oldest son he had a “big surprise” for him moments before he attacked his two boys with a hatchet and started a fire that killed them all.

Elizabeth Griffin-Hall, the state social worker who brought Charlie and Braden Powell to their father’s Graham-area home Sunday, recalled the moment in an interview aired Thursday by ABC News.

She described the last time she saw Josh Powell as his two young sons excitedly dashed up the walk to the front door, mere steps ahead of her.

“I saw Josh for just one second,” said Griffin-Hall, who was to supervise the boys’ visit. “His eyes caught mine. He had a look in his eyes... it was friendly enough, just kind of sheepish. He shrugged his shoulders at me like that and then he slammed the door.”

She said she didn’t realize Powell had intentionally locked her out, so she knocked on the door and yelled to be let in.

Then she heard Braden, 5, cry out. Griffin-Hall thought he might have bumped his foot on something, so she renewed her plea to be let in so she could help.

She wanted to call Powell’s cellphone but her phone was in the car, so she walked the 10 steps or so to her vehicle. As she backed away, Griffin-Hall smelled gasoline.

“A few minutes later, the house exploded,” she said.

Authorities say Powell used a hatchet on his young sons before setting their rental house ablaze. He doused the home with 10 gallons of gas so flames quickly engulfed the house as Griffin stood nearby, calling her supervisor and 911 for help.

Griffin-Hall said she was surprised because she had supervised previous visits with Powell and his sons and there had been no problems.

Powell lost custody of Charlie and Braden last fall, when his father, Steve Powell, was charged with child pornography and voyeurism. Powell and his sons were living in his father’s South Hill house at the time.

Four days before Powell killed himself and the boys, a Pierce County judge ruled against returning the boys to Powell until he passed a psychosexual examination, which would have included a polygraph test.

Powell was a person of interest in the murder of his wife, Susan Cox Powell, who disappeared from their Utah home in 2009.

He adamantly denied involvement in his wife’s disappearance and said he had taken Charlie and Braden - then 4 and 2 - on a midnight camping trip in below-freezing weather and returned to find his wife missing. They moved to the Puyallup area shortly after.

Griffin-Hall said Charlie and Braden loved being with their father.

“One of them said what he wanted to do was go home and live with his daddy,” she told ABC, adding the boys would “light up” during visits with Powell.

They had been living with their maternal grandparents but had court-ordered supervised visits with their father.

Griffin-Hall told ABC she doesn’t think she could have saved the boys.

“How this happened is that Josh Powell was really, really evil. I couldn’t have stopped him,” she said. “I did everything I was supposed to do. I did everything right and the boys are still dead.”